I thought it was established in Toy Box that to the people of Quadratum, Sora and his universe is a "video game" and therefore fictional?
It seems that to Kingdom Hearts, Quadratum is a video game and "fictional" and to Quadratum, Kingdom Hearts is a video game and "fictional." I don't think one is a true "reality" due to this paradox.
I took this as Young Xehanort going along with what Rex believed - that Sora was a Yozora action figure. It is interesting though that in the "split" Toy Box world, Sora ends up in the side of Toy Box where the fictional stuff (like Buzz's laser) is now real. Which makes the Toy Box world even more interesting and hopefully, Nomura explains a deeper reason for Young Xehanort to visit the world. Young Xehanort researching how hearts' connections can bring toys to life is relevant to Xion and the other replicas, but honestly I feel like Young Xehanort was there for something else, something to do with Quadratum and understanding the nature of unreality vs. reality. I can imagine that Master Xehanort probably assumed he would win the X-Blade and was thinking about after beating Sora, what he was going to do with unreality. I mean, the very fact that the toys can be brought to life by Andy's imagination pretty much shows that in the Toy Box world, imagination or "fiction" has some kind of power, even though it's not the unreality side.
This whole thing has me very intrigued to see what exactly Game Central Station is. In KHUX, it's confirmed to not be a data/memory world, but an actual world. But if it's a video game, then is it a portal to Quadratum or somehow connected to Quadratum? I would also lump 100 Acre Wood in there because it's a storybook. Maybe there's some way to create "pockets" of unreality in reality (Sora's reality)? Maybe that's how the Foretellers and maybe even Luxu hid for all these years? What if Toy Box (the split version), 100 Acre Wood, and Game Central Station are pockets of unreality that can exist in Sora's reality? Quadratum can also be a "pocket", just a very large one. At the end of the day, unreality comes from "fiction" which HAS to come from someone's imagination in Sora's reality, right? Otherwise, there would have been no one to create the fiction and hence it would never exist...
OK just gave myself a headache and it's worse than the usual KH headache lol.